Abstract: Venous thromboembolism (VTE) is the third most common cardiovascular disease in the United States. Venous stasis, which can occur during prolonged periods of inactivity, is a known risk factor for VTE; however, the relationship between physical activity and VTE is inconsistent and guidance regarding physical activity after a VTE is lacking. The overarching goal of the proposed scientific aims is to further our understanding of the associations of physical activity and sedentary behavior in relation to: (1) incident VTE risk, (2) short-term VTE risk following physical activity, (3) long-term prognosis following an incident VTE, and (4) hemostatic factor levels, which are associated with thrombotic event risk. This study will leverage existing data from the robust Women?s Health Initiative and the Nurses? Health Studies I and II cohorts in addition to newly collecting data in the setting of a case-crossover study. Innovative aspects of the proposed work include the evaluation of accelerometer-measured physical activity and sedentary behavior in relation to VTE risk, the evaluation of VTE risk following physical activity using case-crossover methodology, the evaluation of physical activity and sedentary behavior with long-term prognosis among VTE survivors, and the evaluation of usual physical activity in relation to hemostatic parameter measures. Not only are the proposed scientific aims of